Add ability to sync multiple files

[corentin-regent]

Purpose

This PR adds the ability to synchronize the repository with labels defined in multiple labels.toml files.

Use case

I created Poetrel, a GitHub action that (among other features) bumps the project version based on the provided PR label. For example, if the PR is labeled with then the major version will be bumped when merging.

Because there are quite a few labels that are supported, I would like to provide an action that would synchronize the user's repository labels with Poetrel, using a labels.toml file defined in my repository. However I would also like to keep all the labels existing in the user's repo.

So the solution is to have a labels.toml file in the Poetrel repository defining the Poetrel labels, another labels.toml file in the user's repository defining all other labels, and run labels sync with both files, which would combine all labels before pushing them to the repo.

Regression

Note that I removed the functionality for writing the labels back to the labels.toml file after the sync, as implementing it with multiple files would require some refactoring, and I feel like it is not really necessary.

Please let me know if this would be an issue for you.